Azure blobs: Simplify error conversion by addition of to_download_error

[arpad-m]

There is a bunch of duplication and manual Result handling that can be simplified by moving the error conversion into a shared function, using map_err, and the question mark operator.

[github-actions]

2292 tests run: 2176 passed, 0 failed, 116 skipped (full report)

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Code coverage (full report)

• functions: 52.9% (8263 of 15613 functions)
• lines: 80.6% (48192 of 59786 lines)

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_{The comment gets automatically updated with the latest test results
fdf30a8 at 2023-10-16T21:53:05.128Z :recycle:}